|M.Sc Student||Brunstine-Townley Alex|
|Subject||Examination of Proton Conductivity in Protein Materials|
and Peptides Probed via Excited-State Proton
|Department||Department of Chemistry||Supervisor||Dr. Nadav Amdursky|
|Full Thesis text|
Protein-based materials present a natural choice for bioelectronic applications. The materials exhibit hydrogen-bond networks composed of hydrated amino acids that show promising application as protonic conductors. This phenomenon could be manipulated to create new devices for scientific and therapeutic ventures, bridging the gap between man and machine.
Light-activated Brønsted-Lowry acids and bases, better known as photoacids and photobases, undergo excited state proton transfer. As such, protons can be released or captured, by application of light.
The main scientific question of the thesis asks if it is possible to modulate charge carrier density, and consequently ionic conductivity, across protein materials using light. Covalently doping a protein material with photoacids enables a light-induced proton release upon optical excitation; this proton released to the mat is intended to act as a charge carrier to improve conductivity of the material. The main rationale of covalent modification stems from literature studies that show weak modulation of proton conductivity via simple mixing of components. It is also surmised that the magnitude of effect can be modulated by light intensity, thus realizing a light-gated field effect transistor.
An analogous study focuses on a device using photobases, featuring excited state proton capture. One of two propositions have been formulated concerning the mechanism of the photobase 1) captures protons from water environment to create localized void of charge carriers, thus decreasing charge carrier density in a manner complementary to the photoacid, or 2) deprotonates water to generate hydroxide charge carriers and subsequently increase the charge carrier density. The photobase-modified material represents the first of its kind.
Results show the photoacid mat releases protonic charge carriers under irradiation to produce over 5.4 ± 1.3 folds increase in conductivity to a final value of 0.706 ± 0.215 mS/cm brought on by a 3.98-fold increase in charge carrier density. The photobase modified mat exhibits a 3.62 ± 0.51 fold increase in conductivity as well. The photobase is proposed to abstract its proton from water, thus creating additional hydroxide charge carriers as evidenced by a rise in conductivity. Further spectroscopic measurements confirm excited state proton transfer is occurring, with 100 ps timescales that correlate with expected values on materials.
In summary, the devices were successfully fabricated and show promising results. The first freestanding photoacid-modified material has been synthesized, with covalent binding showing a much stronger response over simple mixing. In addition, the first photobase-modified material has been fabricated and was shown to increase conductivity upon photoexcitation. Indeed, it is shown conductivity of both materials can be modulated by irradiation with light. The photoacid mat was fully characterized, while additional experiments are needed to explore the mechanism of the photobase mat. Initial attempts to create a device and troubleshooting considerations are explicated. Protein-based materials proved themselves important biopolymers for various applications in the biomedical field. Our new ability to tune ionic conductivity of the material by simple light activation might find itself highly useful for biomedical applications, such as tissue engineering or drug delivery, as well as for energy-related applications like photoelectrochemical cells.